1. Field of the Invention
This invention relates to a radiation shield for covering a portion of a radiation detection camera's field, thereby limiting radiation detection to that from a region which is of interest to the camera user and to allow the camera to better resolve structure in the region of interest by relieving it of the need to process data from background regions of no interest.
A gamma or scintillation camera is typically used in diagnostic analysis of certain parts of the human body. A radioactive substance is administered to a patient and by determining the local and intensity of the radiation as it is emitted from the body, an image of internal structure of the body is obtained.
Typically, the camera remains stationary with respect to the patient while a representation of the spatial distribution of radioactivity is developed. With many of these cameras, a relatively large disk shaped scintillation crystal is positioned to be stimulated by radiation emitted from the patient. In most cameras, a collimator is interposed between the patient and the crystal so that, for example, with a parallel hole collimator, the rays striking the crystal are all generally perpendicular to it.
The crystal scintillates as it converts gamma energy impinging on it to light energy. The light is conducted through a suitable light pipe, to an array of phototubes. When a photo tube is stimulated by light generated in a crystal by a scintillation an electrical signal is emitted which is proportional to the intensity of light energy received by that tube. When a scintillation causes several of the photo tubes to emit signals, these signals are emitted concurrently and are then summed to provide a signal known as the "Z" signal. This "Z" signal is conducted to a pulse height analyzer to determine whether the signal reflects the currents of a so-called photo peak event of the isotope which has been administered to the patient. That is, the "Z" signal is of appropriate strength to reflect the conversion of the energy of a gamma ray emitted from the administered isotope to light energy by the crystal.
Summing and ratio circuits are also provided which develop what are known as "X" and "Y" signals. These "X" and "Y" signals cause a dot to be produced on the screen of the oscilloscope at a location corresponding to the location of the detected scintillation. Thus, the oscilloscope dots are displaced relatively, each at a location corresponding to the location of the corresponding scintillation in the crystal and the oscilloscope dots are integrated to produce an image. Suitable circuits for producing an oscilloscope image of spatial distribution of a radioactive isotope are known.
The photo tubes, the circuits, and the oscilloscope function in a unit to provide a light amplifier such that each dot produced on the oscilloscope represents a scintillation. Through the use of a persistence screen on the scope, or a photographic camera, these dots are integrated to produce an image.
While it is known that certain radioactive substances tend to localize in a given tissue of the body, those substances may collect in areas that are not of interest to the physician for the study being conducted. Thus, in a heart study, for example, radioactive material may collect in the lungs. In observing the image from the gamma camera, the physician is only interested in radiation images coming from the heart and has no interest in radiation coming from the lungs. It would therefore be advantageous to eliminate signals coming from the lungs or any other area of the body not of interest.
It would also be beneficial to block out unwanted radiation in order to improve spatial resolution in the gamma camera image by enhancing the information density for improved image quality. When a scintillation occurs in the detecting crystal it produces an electrical signal. During the time in which the imaging electronics is responding to one radiation count, it cannot process signals indicating the presence of another photon impinging upon the photon crystal. This down or dead time is primarily a result of the processing circuitry's inability to handle two different signals simultaneously. If radiation coming from an area not of interest to the physician causes the scintillation crystal to produce a signal, the radiation coming from the region of interest cannot be processed by the scintillation counter electronics during the ensuing "dead" time. This down time or period in which meaningful radiation detection cannot occur results in a loss of spatial resolution in the final image of radiation in the region of interest.
In studies where the radioactive count rate is measured, the electronic circuitry down time can adversely affect count rate accuracy. A cardiac bypass study, for example, provides a quantitative measure of a subject's heart output. If radiation emitted from outside the heart area is processed, the efficiency in recording scintillations caused by radiation coming from the heart is diminished. This inefficiency occurs because the imaging electronics cannot process all signals coming from the heart if it must process signals from outside that region of interest.
2. Prior Art
In the past, this problem was dealt with by requiring the patient to wear a specially designed leaded garment, having appropriately configured openings, to shield radiation coming from areas of the body not of interest. This technique was awkward and required many variously designed heavy shield garments for various regions of interest corresponding to parts of the body.